1. Field of the Invention
This invention relates to polyvinyl alcohol microgels and their use as paper and paperboard sizes, in paper and paperboard coatings, as textile warp sizes, as adhesives, and as soil and dust stabilizers.
2. Description of the Prior Art
Certain materials such as starch, sodium carboxymethyl cellulose, sodium alginate, and polyvinyl alcohol, with or without the addition of clay, have been used as surface sizes for paper and paperboard. By "surface size" is meant a material which is applied to the surface of the paper or paperboard, thereby forming a barrier which prevents or retards wicking and absorption of ink and other fluids into the paper of paperboard. Surface sizes are to be distinguished from internal sizes which are added in the process of making the sheet.
Although polyvinyl alcohol is many times more expensive than starch on a per pound basis, it has been shown that polyvinyl alcohol can perform as well as, or in many cases better than, starch as a surface size for paper or paperboard on an equal cost basis. However, in most use areas the improvement in performance has not been sufficient to cause former starch users to switch to polyvinyl alcohol. At present polyvinyl alcohol is definitely preferred to starch only in specialty areas where quality is important.
There is evidence that polyvinyl alcohol solutions readily penetrate into paper and paperboard. It is felt that even better performance for polyvinyl alcohol would be demonstrated if its "hold out" characteristics could be improved; that is, if penetration of polyvinyl alcohol size solutions into paper and paperboard could be substantially reduced or eliminated.
It is known that polyvinyl alcohol can be gelled by the addition of various gelling agents such as Congo Red, Direct Orange 8 (Colour Index 22120), Direct Green 12 (Colour Index 30290), borax, and various compounds of aluminum, chromium, copper, iron, titanium, vanadium and zirconium. In U.S. Pat. Nos. 2,720,468; 3,318,856; and 3,492,250 the use of various organo-titanium complexes to gel polyvinyl alcohol is taught. Since these gels are soft-to-rigid solids they are not useful as surface sizes for paper or as textile warp sizes.
A two step-sizing technique has recently been introduced which utilizes the reaction between polyvinyl alcohol and borax to control the penetration of the size solution into paper or paperboard. In this process the paper or paperboard is first pretreated with a borax solution, followed by application of the polyvinyl alcohol size solution. At borax levels about 2% by weight, based on the polyvinyl alcohol, a crosslinking reaction resulting in the formation of a rigid gel, takes place on the surface of the sheet, thus preventing excess penetration of the polyvinyl alcohol size. Although this process can reduce the cost of polyvinyl alcohol sizing, it has not been widely accepted because it introduces an additional step in the sizing operation.